1. Field of the Invention
The invention deals with the field of three dimensional printing, more specifically the printing of relief features on a rotating cylindrical support using a fluid depositing apparatus such as an inkjet printhead. Even more specifically, the invention deals with the field of creating a flexographic print master on a rotating drum by a depositing printhead that moves in a slow scan direction and deposits curable liquid such as a UV-curable liquid.
2. Description of the Related Art
In flexographic printing or flexography a flexible cylindrical relief print master is used for transferring a fast drying ink from an anilox roller to a printable substrate. The print master can be a flexible plate that is mounted on a cylinder, or it can be a cylindrical sleeve.
The raised portions of the relief print master define the image features that are to be printed.
Because the flexographic print master has elastic properties, the process is particularly suitable for printing on a wide range of printable substrates including, for example, corrugated fiberboard, plastic films, or even metal sheets.
A traditional method for creating a print master uses a light sensitive polymerisable sheet that is exposed by a UV radiation source through a negative film or a negative mask layer (“LAMS”-system) that defines the image features. Under the influence of the UV radiation, the sheet will polymerize underneath the transparent portions of the film. The remaining portions are removed, and what remains is a positive relief print plate.
In the unpublished applications EP08172281.1 and EP08172280.3, both assigned to Agfa Graphics NV and having a priority date of 2008-12-19, a digital solution is presented for creating a relief print master using a fluid droplet depositing printhead.
The application EP08172280.3 teaches that a relief print master can be digitally represented by a stack of two-dimensional layers and discloses a method for calculating these two-dimensional layers.
The application EP08172281.1 teaches a method for spatially diffusing nozzle related artifacts in the three dimensions of the stack of two-dimensional layers.
Both applications also teach a composition of a fluid that can be used for printing a relief print master, and a method and apparatus for printing such a relief print master.
FIG. 1 shows an embodiment of such an apparatus 100. 140 is a rotating drum that is driven by a motor 110. A printhead 160 moves in a slow scan direction Y parallel with the axis of the drum at a linear velocity that is locked with the rotational speed X of the drum. The printhead jets droplets of a polymerisable fluid onto a removable sleeve 130 that is mounted on the drum 140. These droplets are gradually cured by a curing source 150 that moves along with the printhead and provides local curing. When the relief print master 130 has been printed, the curing source 170 provides an optional and final curing step that determines the final physical characteristics of the relief print master 120.
An example of a printhead is shown in FIG. 3. The printhead 300 has nozzles 310 that are arranged on a single axis 320 and that have a periodic nozzle pitch 330. The orifices of the nozzles are located in a plane that corresponds with the nozzle plate.
FIG. 2 demonstrates that, as the printhead moves from left to right in the direction Y, droplets 250 are jetted onto the sleeve 240, whereby the “leading” portion 211 of the printhead 210 prints droplets that belong to a layer 220 having a relatively smaller diameter, whereas the “trailing” portion 212 of the printhead 210 prints droplets on a layer 230 having a relatively larger diameter.
Because in the apparatus in FIGS. 1 and 2 the linear velocity of the printhead in the direction Y is locked with the rotational speed X of the cylindrical sleeve 130, 240, each nozzle of the printhead jets fluid along a spiral path on the rotating drum. This is illustrated in FIG. 4, where it is shown that fluid droplets ejected by nozzle 1 describe a spiral path 420 that has a pitch 410.
In FIG. 4, the pitch 410 of the spiral path 420 was selected to be exactly equal to the length of the nozzle pitch 430 of the printhead 440. In a more general preferred embodiment the pitch of the spiral path is an integer multiple “N” of the nozzle pitch. In such a case there are N interlaced spiral paths.
A prior art system such as the one depicted in FIG. 2 and FIG. 4 suffers from an unexpected problem.
The droplets that are ejected by the nozzles of the printhead 210, 440 have a finite velocity while they travel to their landing position. As a result it takes some time for them to reach their landing position on the rotating drum. The effect can be described as “landing position lag”. This landing position lag—by itself—poses no problem. However, in the prior art system shown in FIG. 2, the nozzles near the leading edge of the printhead eject droplets that land on a layer of the print master having a relatively smaller diameter, whereas the nozzles near the trailing edge of the printhead eject droplets that land on a layer having a relatively larger diameter. The effect of this is that the droplets ejected by nozzles near the leading edge of the printhead be subject to more landing position lag compared with droplets ejected by nozzles near the trailing edge of the printhead. This results in a distortion of the three-dimensional grid that makes up the relief print master, since droplets that are intended to be stacked on top of each other in the different layers will be shifted relatively to each other. This weakens the matrix of droplets that make up the relief print master.